Various optical network applications require an optical beam exiting a fiber to be expanded. Consequently, a variety of fiber “terminations” (structures for terminating a fiber and expanding the beam exiting it) have been developed to address this requirement. One general type of fiber termination produces an at least substantially collimated optical beam. Examples of this type are a ball-lensed collimators, e.g., of FIG. 1A, and gradient index of refraction, or “GRIN,”-lensed collimators, e.g., that of FIGS. 1B and 1C.
The ball-lensed collimator of FIG. 1A accepts a fiber 100 proximate the focal plane of the ball lens 104. The collimator in some cases has a protective transparent plate 106. Air interfaces (unreferenced) lie between the end of the fiber 100, the surface of the ball lens 104 and the surface of the transparent plate 106. Unfortunately, Fresnel back reflections occur not only at the air interface between the end of the fiber 100 and the surface of the ball lens 104 but also the air interface between the ball lens 104 and the transparent plate 106. These Fresnel back reflections are typically −10 dB to −15 dB, which have become unacceptable.
To reduce the back reflections, an antireflection (AR) coating 102 may be added to one or more of the end of the fiber 100, the surface of the ball lens 104 and the surface of the transparent plate 106. These typically reduce the reflection to a range from −24 dB to −45 db, depending on the optical bandwidth and the type and quality of AR coating applied. Unfortunately, many of these AR coatings 102 cannot withstand exposure to extreme temperatures, chemicals or abrasion.
The orthogonal GRIN-lensed collimator of FIG. 1B is becoming increasingly accepted. The collimator of FIG. 1B accepts a fiber 100 proximate a quarter-pitch GRIN lens 108. Unfortunately, the air interface between the fiber 100 and the quarter-pitch GRIN lens 108 and at the exit of the quarter-pitch GRIN lens 108 exhibit Fresnel back reflections of −48 dB and −12 dB, respectively. As above, AR coatings may be employed to mitigate the back reflections. Unfortunately, an epoxy 109 is required to affix the fiber 100 to the quarter-pitch GRIN lens 108. Consequently, the resulting epoxy interface can increase the Fresnel back reflection to −25 dB.
The beveled GRIN-lensed collimator of FIG. 1C addresses some of the shortcomings of the orthogonal GRIN-lensed collimator of FIG. 1B. The quarter-pitch GRIN lens 108 is provided with bevels 110. The fiber 100 is beveled to match the bevel 110 such that the Fresnel back reflection generated epoxy interface does not substantially re-enter the fiber 100. The bevel 110 at the opposing end of the quarter-pitch GRIN lens 108 serves the same purpose, namely to keep the Fresnel back reflection from substantially reentering the fiber 100. Unfortunately, this causes a beam 111 carried in the fiber 100 to exit the quarter-pitch GRIN lens 108 at a nonzero angle 112 with respect to the optical axis of the quarter-pitch GRIN lens 108. This angle is typically on the order of 8°. Also disadvantageous is the fact that while the back reflections have been rerouted, the losses they represent remain.
Many specifications today, such as MIL-DTL-38999, specify a maximum of −35 dB back reflection. As the ability to measure back reflection has increased to −50 dB and perhaps beyond, demand is building for waveguide terminations having less than a −50 dB back reflection, while exhibiting a relatively low insertion loss.